Because of their small density, good ductility and toughness, easy molding processing, good electrical conductivity, thermal conductivity, and other physical properties, aluminum alloys have been widely used in modern aviation, aerospace, electric power, electronics, petroleum, chemicals, building materials, transportation, light industry, and national defense industries. However, as a typical amphoteric metal, aluminum is prone to corrosion in both acid and alkaline environments, which limits the applications of aluminum and its alloys. In addition, the surface hardness and wear resistance of aluminum alloy are poor. Usually, surface treatment is used to overcome these shortcomings and improve the product performance. The main methods include: oxidation treatment, electroplating, plastic coating, and spray painting.
Electroplating is the most widely used method. On the one hand, it can retain or even strengthen the main physical properties, such as electrical conductivity, thermal conductivity, mechanical performance, etc. On another hand, it can avoid the corrosion of acid and base environments, and expand the application of aluminum alloys. Furthermore, electroplating can improve the decorative effect on the surface of the aluminum alloys.
In general, two electroplating processes are used for aluminum alloys. For example, the alloys are pre-treated, dipped into a zinc-containing solution, coated with a nickel layer by a chemical method, electroplated by a copper layer, electroplated by a nickel layer, and electroplated by a chromium or gold layer. The other process includes pre-treating, dipping into a zinc-containing solution, electroplating a copper layer from a cyanide copper solution, electroplating a copper layer from a pyrophosphate copper solution, electroplating copper from an acid copper solution, electroplating a nickel layer, and electroplating a chromium or gold layer. These processes have shortcomings, such as poor adhesion of the electroplated layers, high costs, difficult treatments of waste water, and serious environmental pollutions. The main disadvantages of nickel electroplating include that exterior appearance may be affected by the nickel deposition, and nickel would cause some people to have allergic reactions. The current nickel-free electroplating processes include cyanide Cu—Sn alloy electroplating, cyanide-free Sn—Co alloy electroplating, cyanide-free Sn—Zn—Co alloy electroplating, cyanide-free Sn—Zn alloy electroplating, and so on. The cyanide-free alloys do not contain cyanide, but the color of the coatings is not bright. The technique of cyanide Cu—Sn alloy electroplating is relatively mature, but the high toxicity of cyanides limits its application, and cyanide may cause serious environmental pollutions.
Therefore, a nickel-free and cyanide-free process is desirable to electroplate aluminum alloys.